Grefsen is a neighbourhood in the city of Oslo, Norway.
Grefsen was a part of the municipality of Aker before the Second World War, later incorporated into Oslo. Together with Kjelsås, Grefsen then formed the borough Grefsen-Kjelsås until 1 January 2004, when they became part of the new borough of Nordre Aker.
The railway line Gjøvikbanen goes through the area, but Grefsen Station is actually located closer to the neighbourhood of Disen. The Kjelsås Line also runs through the area. The local sports field Grefsen stadion is the home field of Kjelsås IL.
There are three schools in the area, Grefsen Elementary School (year 1-7), Engebråten Middle School (year 8-10) and Nydalen High School (year 11-13). Nydalen High School used to be Grefsen High School until to 2014, when it was renovated and changed name.
The neighbourhood is named after the old farm Grefsen (Norse Grefsin, from *Grefsvin). The first element is *grefs 'steep hillside' (derived from the verb grafa 'dig (out)'), the last element is vin f 'meadow'. The old farm had Akerselva as its western border, and the hillside/riverside down to the river is very steep.
The adjacent hill Grefsenåsen is named after the farm.